over sized pistons / rings

the compression on my first cylinder is down and when i did the head gasket i noticed the cylinder was scored (ridge) along the top and my exhaust valves were burnt also i get oil coming from the breather , however being a novice mechanic i just put it all back together, now i have been told that i need to rebore the engine, £80 , but where would i get over sized pistons or can i get oversized piston rings to fit on my current ones, also can anyone tell me where to buy them and what kind of prices am i looking at.